1. Field of the Invention
The present invention relates to variable frequency oscillators. More particularly, the present invention relates to controlling amplitude of a signal from an amplifier providing the output of the variable frequency oscillator as oscillator frequency increases when the amplifier is operating near its process limits where amplifier gain decreases significantly with increasing oscillator frequency.
2. Description of the Related Art
With variable frequency oscillators, and particularly variable frequency oscillators using integrated circuit components, output amplitude will typically vary significantly with frequency over at least a portion of the frequency operation range. Output signal amplitude will vary due to factors such as slew rate, transconductance (gm), temperature, and power supply voltage. Typically a goal is to maintain constant amplitude with all these factors.
For digital applications, a variable frequency oscillator typically serves as a clock signal source so that the output of its oscillator will be amplified, but then limited to obtain a square wave signal. The amplification in such digital applications is regarded as nonlinear amplification, because the limiting enables variations in frequency to occur without changes in amplitude.
For analog applications, oscillators are also utilized, but precise linear amplification is needed, unlike with digital applications. Such linear amplification, however, is difficult to control against the significant number of factors mentioned above when frequency is varied, particularly when the oscillator frequency approaches its limits due to limitations in integrated circuit process technology.
When the oscillator in a variable frequency oscillator is approaching its frequency limits due to limitations in integrated circuit technology, it is almost certain that the amplifier used to output a signal from the variable frequency oscillator is also near its operation limits. In other words, the operating frequency of the amplifier will be beyond its dominant pole as shown by the "region of operation" in FIG. 1, which plots typical amplifier gain vs. frequency. With amplifier operation beyond a dominant pole, as shown in FIG. 1, the amplifier gain will typically change in proportion to 1/f due to factors mentioned above, where f denotes frequency of a signal input to the amplifier. Thus, with a variable frequency oscillator using an amplifier operating beyond its dominant pole, output signal amplitude can vary significantly with frequency, contrary to operating conditions typically desired for analog applications.